Re: [PATCH v5 05/25] fs/dax: Create a common implementation to break DAX layouts

[Dan Williams]

Alistair Popple wrote:
> Prior to freeing a block file systems supporting FS DAX must check
> that the associated pages are both unmapped from user-space and not
> undergoing DMA or other access from eg. get_user_pages(). This is
> achieved by unmapping the file range and scanning the FS DAX
> page-cache to see if any pages within the mapping have an elevated
> refcount.
> 
> This is done using two functions - dax_layout_busy_page_range() which
> returns a page to wait for the refcount to become idle on. Rather than
> open-code this introduce a common implementation to both unmap and
> wait for the page to become idle.
> 
> Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
> 
> ---
> 
> Changes for v5:
> 
>  - Don't wait for idle pages on non-DAX mappings
> 
> Changes for v4:
> 
>  - Fixed some build breakage due to missing symbol exports reported by
>    John Hubbard (thanks!).
> ---
>  fs/dax.c            | 33 +++++++++++++++++++++++++++++++++
>  fs/ext4/inode.c     | 10 +---------
>  fs/fuse/dax.c       | 29 +++++------------------------
>  fs/xfs/xfs_inode.c  | 23 +++++------------------
>  fs/xfs/xfs_inode.h  |  2 +-
>  include/linux/dax.h | 21 +++++++++++++++++++++
>  mm/madvise.c        |  8 ++++----
>  7 files changed, 70 insertions(+), 56 deletions(-)
> 
> diff --git a/fs/dax.c b/fs/dax.c
> index d010c10..9c3bd07 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -845,6 +845,39 @@ int dax_delete_mapping_entry(struct address_space *mapping, pgoff_t index)
>  	return ret;
>  }
>  
> +static int wait_page_idle(struct page *page,
> +			void (cb)(struct inode *),
> +			struct inode *inode)
> +{
> +	return ___wait_var_event(page, page_ref_count(page) == 1,
> +				TASK_INTERRUPTIBLE, 0, 0, cb(inode));
> +}
> +
> +/*
> + * Unmaps the inode and waits for any DMA to complete prior to deleting the
> + * DAX mapping entries for the range.
> + */
> +int dax_break_mapping(struct inode *inode, loff_t start, loff_t end,
> +		void (cb)(struct inode *))
> +{
> +	struct page *page;
> +	int error;
> +
> +	if (!dax_mapping(inode->i_mapping))
> +		return 0;
> +
> +	do {
> +		page = dax_layout_busy_page_range(inode->i_mapping, start, end);
> +		if (!page)
> +			break;
> +
> +		error = wait_page_idle(page, cb, inode);

This implementations removes logic around @retry found in the XFS and
FUSE implementations, I think that is a mistake, and EXT4 has
apparently been broken in this regard.

wait_page_idle() returns after @page is idle, but that does not mean
@inode is DMA idle. After one found page from
dax_layout_busy_page_range() is waited upon a new call to
dax_break_mapping() needs to made to check if another DMA started, or if
there were originally more pages active.

> +	} while (error == 0);
> +
> +	return error;

Surprised that the compiler does not warn about an uninitialized
variable here?